1. Field of the Invention
The present invention relates to multi-circuit braking systems having hydraulic brake circuits for motor vehicles.
2. Description of Related Art
In the field of motor vehicle construction, hydraulic brakes have been customary for a long time because of their efficiency and their functional safety. Multi-circuit brake systems take care of a redundancy that still further increases functional safety, and these systems often have several hydraulic brake circuits acted upon by pressure from one or more master brake cylinders. The pressure is generated first by operating a brake pedal and is then reinforced, if necessary. Starting from the master brake cylinder, the wheel brakes have pressure applied to them and are operated thereby, often controlled by valves.
It is also known that one may unload the brakes of a vehicle by additional deceleration elements. In this context, for example, an engine brake may be used, which is also capable of being quantitatively regulated by valve control, or electrical devices may be used such as eddy-current brakes or generators. As generators, generators usually come into consideration which are present in every vehicle, and which are used to charge the starter battery and for the electrical supply of the aggregates of the motor vehicle, or even a larger dimensioned drive motor of the vehicle, such as is present in hybrid vehicles. This drive motor may be driven as a generator in order to charge a battery used for the vehicle drive. Thus, kinetic energy of the vehicle may be stored during deceleration and used again later for accelerating the vehicle. The process of braking support while charging a battery is called recuperative brakes.
In such deceleration processes of the vehicle, however, various problems come up. The driver will usually want quantitatively to regulate suitably the braking effect by operating the brake pedal, in order to be able to optimize the braking distance and the measure of the braking deceleration for the respective traffic situation at hand. At the same time as the vehicle deceleration attained, the driver senses the counteracting force in the brake pedal, so that an iterative control process is made possible for him.
If an additional deceleration aggregate in the form of an operated generator is added to the direct braking effect, its deceleration effect is, on the one hand, not optimally able to be sensed by the driver, and on the other hand, the deceleration effect of the generator is also dependent on many boundary conditions and, not least of all, is changeable with time. For instance, the braking effect of the generator, at changing vehicle speed, may increase or decrease, and when the drive transmission is uncoupled, the deceleration effect at times drops off completely, so that when it is coupled in again, one carefully has to aim for it. An additional problem comes about when the battery is fully charged, since the additionally generated electric power then has to be removed in another way.
The combination of mechanical braking and additional deceleration of the vehicle by other influences is thus manageable for the driver, but not optimally controllable from a comfort point of view. The driver is able to compensate for changes in the vehicle deceleration only by equalization of the braking forces via the brake pedal.
A large number of methods is known from the related art that is supposed to support the driver in the control of brakes while observing comfort aspects and safety aspects. Among such methods are both traction control and dynamic braking during cornering for increasing the driving stability.
Thus, from published German patent document DE 4128087 A1, a brake pressure regulating system for a vehicle is known in which an underbraked rear axle is prevented when braking during cornering. The brake pressure at the front axle is specified by the driver, and the brake pressure at the rear axle is regulated as a function of this.
It is basically also known that one may distribute the brake force in such a way that as strong a braking as possible of the vehicle is achieved while taking grip utilization into account; in this context, it is also possible for wheels that are statically and/or dynamically more heavily loaded to be braked to a correspondingly greater degree as well.
From published European patent document EP 0173954 B1, a system is known in which the brake pressures for individual brakes are ascertained using a reference mass for the vehicle and the nominal deceleration specified by the driver in a stored vehicle-specific characteristic map. The brake pressures ascertained are applied at the brakes and, if necessary, are adjusted if the vehicle deceleration deviates from the nominal value, until the nominal deceleration has been achieved.
From published German patent document DE 3313078 A1, a brake pressure regulation device is known, which ascertains the wear of different wheel brakes and takes it into account, so that an even wear of the individual wheel brakes is achieved in the long term.
From German patent application document DE 102005046606, a braking system is known in which one brake circuit is assigned to one of the axles of a vehicle in each case, so that a traction control device, as well as an electronic stability control device, may be provided in one brake circuit exclusively, which minimizes the constructive overall expenditure.
Finally, from published German patent document DE 10316090 A1, a braking system is known which has a plurality of brake circuits, which basically act hydraulically and act on friction brakes of individual wheels, and which have a generator or an electric drive motor which is able to be operated as generator and may be used for deceleration of the wheels assigned to a brake circuit in addition. A control device is provided in order to optimize the brake force distribution to all individual wheels while taking into account different variables pertaining to driving dynamics.